The invention generally relates to electronic devices, and more particularly to oscillators such as voltage controlled oscillators (VCO""s).
Oscillators such as VCO""s have been used in many applications. One application is to use a VCO in a wireless device, e.g., a cell phone, to provide frequency signals to the transmitter of the cell phone. A conventional VCO is described on pages 227-229 of xe2x80x9cRF Microelectronicsxe2x80x9d published in 1998, by Behzad Razavi, which is hereby incorporated by reference. A simplified version of the conventional VCO is illustrated in FIG. 1, in which feedback and output capacitors are omitted.
In FIG. 1, a VCO 10 comprises a LC tank 14 and an amplifier 18 with a gain of xe2x88x92gm. LC tank 14 includes a pair of inductors L1 and L2 and a tuning capacitor C. A perturbation generator 20 generates magnetic coupling to VCO 10. The perturbation generator can be any conductor that has a large current at the same or a fraction of the frequency of the VCO. This is especially true in applications where the received signals and the local oscillator run at the same frequency (Zero-IF), such as in a wireless device application, e.g., a pager or a cell phone application and the signals are strong (e.g., large current in the transmitter output bondwire). The magnetic coupling causes induced currents, to flow in inductors L1 and L2. The induced currents changes the frequency signals generated by VCO 10. As a result, the output spectrum of the transmitter of a wireless device will be distorted. Consequently, steady communications between the wireless device and other communications devices cannot be reliably established.
One attempted solution to reduce the magnetic coupling is to place the inductors L1 and L2 at such positions that their windings are in opposite directions relative to each other. This allows less coupling from perturbation generator 20 since the induced currents in the inductors are opposite in phase and will offset each other to some extent.
However, the above solution is not practical because the coupling depends on the relative position of the perturbation generator and the inductors. It is feasible only when the inductors can be placed symmetrical with respect to the perturbation generator 20, in which case the inducted currents in inductors L1 and L2 can cancel each other. Otherwise, the magnetic coupling will be different on the two sides and the induced currents are thus different on the two sides. Symmetrical placement is not always possible or practical and does not provide flexibility due to the large sizes of the inductors or multiple perturbation sources.
Therefore, there is a need for an oscillator, e.g., VCO that can provide more stable frequency signals in the presence of strong magnetic coupling.
The present invention provides an oscillator that can supply more stable frequency-signals in the presence of strong magnetic coupling.
According to, one embodiment of the invention, an oscillator is presented and comprises an amplifier and an LC circuit connected to the amplifier. The LC circuit includes a capacitor having two ends, and first and second pairs of inductors, with the two pairs connected to the two ends of the capacitor, respectively.
According to one aspect of the embodiment of the invention, the first pair includes first and second inductors and the second pair includes third and fourth inductors. Further, the first and second pairs are arranged such that the first and fourth inductors are on a first side and the second and third inductors are on a second side.
According to another aspect of the embodiment of the invention, the inductors are arranged such that a center between the first and second inductors substantially overlap a center between the third and fourth inductors. Assuming the magnetic coupling to each pair of inductors is at the center of the two inductors because of center of gravity, it is possible to make the two pairs of inductors look symmetrical to an external perturbation generator without regard to the relative position of the perturbation generator.
According to a further aspect of the embodiment of the invention, the first and second inductors are arranged such that the first and second inductors have their magnetic fields in opposite directions. Moreover, the third and fourth inductors are arranged such that the third and fourth inductors have their magnetic fields in opposite directions. In such arrangement, induced currents in the inductors cancel each other, thus minimizing the output frequency signals of the oscillator.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.